POWAY, Calif. (Reuters) - A 19-year-old man accused of opening fire on Sabbath worshippers in a deadly shooting rampage at a Southern California synagogue is believed to have acted alone, without help from any organized group, authorities said on Sunday.

Police say the gunman walked into the suburban San Diego synagogue late on Saturday morning, the last day of the week-long Jewish holiday of Passover, and killed one woman and wounded three other people inside, using an assault-style rifle.

According to authorities, the suspect, John Earnest, then fled in a car before calling police to hand himself in.

"We believe he acted alone and without outside support in carrying out the attack," San Diego County Sheriff Bill Gore said in a statement.

The suspect, who is also under investigation for a mosque arson, has been booked into custody on one count of murder in the first degree and three counts of attempted murder in the first degree, Gore said.

At an emotional news conference on Sunday, Rabbi Yisroel Goldstein choked back tears as he spoke of the slain victim, Lori Kaye, whom he had known for 25 years.

"She was a pioneer member from our congregation," he said, his left arm in a sling and his right hand bandaged. "Lori had unconditional love for all."

Goldstein had surgery at a hospital after his right index finger was blown away by the gunman. He recounted how after he saw the carnage, he grabbed a prayer shawl, wrapped it around his bloody hands and addressed the congregation.

"We are a Jewish nation that will stand tall, we will not let anyone or anything take us down – terrorism like this will not take us down," Goldstein said he told the worshippers.

The other two wounded in the attack on the Congregation Chabad temple in the town of Poway, about 23 miles (37 km) north of San Diego, were an 8-year-old Israeli girl and her uncle. Their family had moved to the United States in search of a safer life after their home in Sderot on the Gaza border was hit several times by Palestinian rocket attacks.

The sheriff said they were struck by shrapnel but were "doing well" at a hospital.

Goldstein said U.S. President Donald Trump had called him to offer condolences on behalf of the American people.

MOSQUE FIRE INVESTIGATED

After the shooting, the suspect fled in a car, escaping an off-duty U.S. Border Patrol agent who shot at the getaway vehicle but missed the suspect. The suspect pulled over and surrendered to police officers a short time later.

The teenage suspect, who had no prior criminal record, is the apparent author of a "manifesto" who claimed to have set a nearby mosque on fire last month and professed drawing inspiration from the gunman who killed nearly 50 people at two mosques in New Zealand.

Gore said on Saturday that the police and FBI were investigating Earnest's possible involvement in the March 24 pre-dawn arson fire at the Islamic Center of Escondido, a town about 15 miles (24 km) north of Poway. No one was hurt at the mosque fire.

Saturday's bloodshed in Poway unfolded six months to the day after 11 worshippers were killed and six other people wounded by a gunman who stormed the Tree of Life synagogue in Pittsburgh yelling: "All Jews must die."

The assailant in that massacre, said to be the deadliest attack ever against Jews on U.S. soil, was also arrested.

The Passover violence came amid an upsurge in reports of anti-Semitism nationwide and abroad and followed a recent spate of deadly attacks on places of worship around the world.

Suicide bombings during Easter Sunday services at several churches in Sri Lanka killed more than 250 people. A gunman who opened fire at two mosques in Christchurch, New Zealand, on March 15 left 49 people dead and more than 40 wounded, some as they knelt in prayer.

A rambling, violently anti-Semitic, anti-Muslim screed written by an individual calling himself John Earnest was found posted online, with links to the content posted on a far-right forum on the internet message board 8chan.

In that letter, the author also claimed credit for the Escondido mosque arson, which was put out by congregants who were sleeping inside and woke up to the smell of smoke. Local media at the time reported that a message scrawled on the driveway of the mosque mentioned the New Zealand massacre.

Earnest was enrolled at California State University in San Marcos. In a statement, the university's president said the school was "dismayed and disheartened" to learn Earnest was a student and was working with the sheriff's department.

Earnest attended Mt. Carmel High School in San Diego, where his father taught physics before retiring in 2016, according to the San Diego Union-Tribune newspaper.